Simian sarcoma virus (SSV) deletion mutants were constructed from a molecular clone containing the entire infectious provirus. Transfection analysis of these mutants localized the SSV transforming gene to a small region of the viral genome encompassing its cell-derived sequence (v-sis). Antiserum to a peptide synthesized on the basis of the predicted amino acid sequence of the SSV transforming gene detected a 28,000-dalton protein that was specifically expressed in SSV transformed cells and that corresponded in size to that predicted from the v-sis coding sequence. The v-sis gene product designated p28sis was not a phosphoprotein, nor did it possess detectable protein kinase activity. These findings distinguish p28sis from a number of other retroviral onc proteins.